The present invention relates generally to an instantaneous frequency measurement (IFM) receiver to measure frequency and angle of arrival (AOA) of multiple signals.
An IFM receiver can have very wide bandwidth, and generate fine frequency information on short pulsed signals. It also has small size and light weight. Although it has all the desired properties required for EW (Electronic Warfare) applications, it has one major deficiency; that is, the IFM receiver can not process simultaneous signals. Worse yet, under a simultaneous signals condition, the receiver may report erroneous frequency data which will confuse the processor following the receiver. Dr. Donald Tufts invented an approach using an IFM receiver to measure the frequencies of simultaneous signals through Prony's method, which is a mathematical method to solve certain equations. This approach is studied through an Air Force contract F33615-85-C-1720. (See the final technical report AFWAL-TR-86-1131, "Fast Measurement of Signal Parameters in the Presence of Signal-Like Interference, Simultaneous Signal Processing", by S. M. Risteen, submitted, Mar. 10, 1987 (Dr. Tufts was a consultant not named in the report)). However, the AOA problem was not properly solved during the contracting period. A closed form solution is needed (reference section 9.3.3. of the said final report).
United States patents by applicant James B. Y. Tsui sole or et al for IFM and other EW receivers include (1) U.S. Pat. No. 4,194,206 issued Mar. 18, 1980 for an Instantaneous Frequency Measurement (IFM) Receiver With Capability to separate CW and Pulsed Signals; (2) U.S. Pat. No. 4,336,541 issued Jun. 22, 1982 for Simultaneous Signal Detector for an Instantaneous Frequency Measurement Receiver; (3) U.S. Pat. No. 4,426,648 issued Jan. 17, 1984 for Simultaneous Signal Detection for an IFM Receivers by Detecting Intermodulation Products; (4) U.S. Pat. No. 4,547,727 issued Oct. 15, 1985 for Simultaneous Signal Detection for IFM Receivers by Transient Detection; (5) U.S. Pat. No. 4,633,516 issued Dec. 30, 1986 for an Instantaneous Frequency Measurement Receiver With Digital Processing; (6) U.S. Pat. No. 4,200,840 issued Apr. 29, 1980 for a Dual Detection Scheme for Compressive Receivers; and (7) U.S. Pat. No. 4,644,267 issued Feb. 17, 1987 for a Signal Analysis Receiver With Acousto-Optic Delay Lines. There is also (8) an application (by Shaw, Tsui & Hedge) Ser. No. 215,662 filed Jul. 6, 1988, for PSK Detection Using an IFM Receiver. These patents and application are hereby incorporated by reference. References (2), (3) and (8) show the circuit of a basic IFM correlator as shown in FIG. 1 herein.
Other United States patents of interest are U.S. Pat. No. 3,939,411 to James, which teaches an IFM system capable of measuring pulse signals of differing frequencies emitted by a single source. U.S. Pat. No. 4,532,515 to Cantrell et al teaches a device for measuring AOA by converting received echoes to complex numbers representing the amplitude and phase. U.S. Pat. No. 4,638,319 to Chua teaches a RF system wherein a phase comparison means provides a signal output and bearing angle.